Due to the wide use of portable electric products such as video cameras, mobile telephones, and portable PCs, secondary batteries, which can be used as the driving power sources of these portable electric products, have become increasingly important. This is because lithium secondary batteries have greater energy concentration per unit weight than other secondary batteries, such as conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries. In addition, lithium secondary batteries can be rapidly charged. For all these reasons, use of lithium secondary batteries has significantly increased.
Lithium secondary batteries are classified according to the electrolyte used. Lithium ion batteries use liquid electrolytes, and lithium ion polymer batteries use solid polymer electrolytes. Lithium ion polymer batteries can use completely solid polymer electrolytes, which include no liquid electrolyte, or they may use gel type polymer electrolytes, which include some liquid electrolyte.
Lithium ion batteries, which use liquid electrolytes, can be contained in cylindrical or angular metal cans which are sealed by soldering. These can type secondary batteries have limitations on shape, restricting the design of electric products employing these batteries as power sources. Also, reducing the volume of the electric products is difficult with can type secondary batteries.
To address these limitations, pouch type secondary batteries in which two electrodes, a separator, and an electrolyte are contained in a sealed pouch comprising a film, have been developed as alternatives to can type secondary batteries. FIGS. 1 and 1A illustrate a typical prior art pouch type secondary battery. As shown in FIG. 1A, the pouch of a typical lithium ion polymer battery has a multi-layered structure comprising a polyolefin thermal adhesive layer 15 having thermal adhesive properties which serves as a sealant, an aluminum metal layer 13 that serves as a material for maintaining mechanical strength, a water and oxygen barrier layer, a nylon layer 11, and a protective layer. The pouch is formed by sequentially laminating these layers. Cast polypropylene (CPP) is commonly used as the polyolefin layer.
As shown in FIG. 1, the pouch comprises a container 20 having a cavity 21 and a cover 10 for covering the cavity 21. An electrode assembly 30 is contained within the cavity 21, and comprises an anode 31, a cathode 35 and a separator 33. The anode 31, cathode 35 and separator 33 are laminated and wound to form the electrode assembly 30. Electrode tabs 37 and 38 protrude from the anode and cathode and tape 39 is attached to the portion of each tab that lies on the lip 23 of the container 20 of the pouch.
Pouch type secondary batteries are flexible and can take a variety of shapes, making it possible to manufacture secondary batteries having the same capacity, but having reduced volume and weight. However, unlike can type secondary batteries, the pouches used in pouch type secondary batteries are soft, making the pouch type secondary batteries weak, and decreasing the reliability of the pouch seal. Therefore, pouch type secondary batteries are mainly used with lithium ion polymer batteries using gel type or completely solid polymer electrolytes, rather than with lithium ion secondary batteries using liquid electrolytes, which may leak.
The electrode assembly is positioned in the pouch of the pouch type secondary battery such that a small pouch can have large capacity, as is needed for secondary batteries. Also, reduction of the area of the lip 23 surrounding the container 20 is desired since the lip 23 is not related to battery capacity or battery function. When the lip 23 is reduced, electrode assemblies having greater capacity can be contained in the pouch, resulting in secondary batteries having greater capacity. However, reduction of the lip 23 also reduces the reliability of the seal since the area to which the cover 10 is sealed to the container 20 is reduced.
However, during thermal fusion of the thermal adhesive polyolefin layer of the cover 10 to the thermal adhesive polyolefin layer of the container 20, a portion of the fused polyolefin layer seeps out of the pouch due to the external pressure applied during fusion. Such an occurrence adversely affects the external appearance of the pouch and reduces the thickness of the fused polyolefin layer, thereby reducing the reliability of the seal. Furthermore, enough of the polyolefin layer may seep out of the pouch during sealing to expose the metal barrier layer underneath the polyolefin layer. This occurrence causes a short circuit between the pouch and the electrode tabs.